1. Field of the Invention
The present invention relates to an electric power steering system which employs an electric motor as a drive source which is controlled on the basis of a steering torque.
2. Description of Related Art
Electric power steering systems are conventionally utilized which are adapted to assist a steering operation by transmitting a torque generated by an electric motor to a steering mechanism of a motor vehicle. The electric motor is controlled for driving thereof on the basis of a target current determined depending on a vehicle speed and a steering torque applied to a steering wheel.
However, if an ignition key switch of the motor vehicle is turned on or off with the steering torque being applied to the steering wheel, a driver feels the steering wheel abruptly becoming lighter or heavier upon the start or stop of the electric motor. This deteriorates the upscale image and steering feeling of the motor vehicle. Particularly, where a high power electric motor is employed in an electric power steering system for a larger-scale motor vehicle, the change in the feeling of the steering wheel upon the start or stop of the system is drastic. When the system is deactuated, an abrupt turn-back of the steering wheel may even occur upon the stop of the electric motor.
It is an object of the present invention to provide an electric power steering system which ensures an improved steering feeling at the start and/or stop of the operation of the electric power steering system.
The electric power steering system according to the present invention, which employs an electric motor (M) as a drive source to apply a steering assist force to a steering mechanism (3) mounted on a motor vehicle in accordance with the operation of an operation member (1), comprises: a torque sensor (5) which detects a steering torque applied to the operation member and outputs a torque signal in accordance with the detected steering torque; a motor driving control circuit (14; 70, S4, S14) which outputs a motor driving command signal for driving the electric motor on the basis of the torque signal outputted from the torque sensor; and an output limitation circuit (12, 21; 70, 5, S15) which limits the motor driving command signal in accordance with the magnitude of the steering torque detected by the torque sensor at start or stop of the operation of the electric power steering system. The parenthesized numerals hereinbefore and hereinafter denote corresponding components employed in the embodiments to be described later.
The output limitation circuit may be software-based or may be based on hardware such as analog circuits.
The operation of the electric power steering system is started when an ignition switch is turned on, or when it is judged that the electric power steering system is restored from a malfunction state to a normal state after the operation of the electric power steering system in malfunction is once stopped. The operation of the electric power steering system is stopped when the ignition switch is turned off, or when it is judged that the electric power steering system suffers from some malfunction.
In accordance with the present invention, the motor driving command signal is limited in accordance with the magnitude of the steering torque at the start and/or stop of the operation of the electric power steering system. Thus, an abrupt change in the steering assist force can be suppressed when the electric power steering system is started or stopped with the steering torque being applied to the operation member such as a steering wheel. Therefore, a feeling obtained from the operation member by an operator (driver) can be improved.
In one embodiment of the present invention, the output limitation circuit (12, 21; 70, S5) more heavily limits the motor driving command signal at the start of the operation of the electric power steering system, as the steering torque detected by the torque sensor is large.
Thus, even if the steering torque is great at the start of the operation of the electric power steering system, there is no possibility that a relatively great steering assist force is abruptly generated. Accordingly, the steering assist force is not steeply increased, so that the steering feeling can be improved.
The output limitation circuit preferably sets a limitation value indicative of a degree of the limitation of the motor driving command signal so that the limitation degree does not increase with time (or decreases with time) at the start of the operation of the electric power steering system.
In another embodiment of the invention, the output limitation circuit (12, 21; 70, S15) more heavily limits the motor driving command signal at the stop of the operation of the electric power steering system, as the steering torque detected by the torque sensor is small.
Thus, when the steering torque is great at the stop of the operation of the electric power steering system, a relatively great steering assist force can continuously be generated. Therefore, there is no possibility that the steering assist force is abruptly eliminated, so that the steering feeling can be improved.
The output limitation circuit preferably sets the limitation value indicative of the degree of the limitation of the motor driving command signal so that the limitation degree does not decrease with time (or increases with time) at the stop of the operation of the electric power steering system.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.